The disclosure of the following priority application is herein incorporated by reference: Japanese Patent Application No. 11-206140 filed Jul. 21, 1999.
1. Field of the Invention
The present invention relates to a digital camera and to an image signal processing apparatus in which an image of a photographic subject is stored as electronically compressed image data.
2. Description of the Related Art
A known type of digital camera comprises an image-capturing device such as a CCD which captures an image of a photographic subject via a photographic lens and outputs image data, and an image processing circuit which performs image processing, such as for example white balance adjustment or gamma correction, by adjusting the gain for the image data output from the image-capturing device. In the image processing circuit image processing is performed by calculating parameters such as the R-gain and/or the B-gain for the white balance adjustment, or the gradation curve for the gamma correction or the like, according to a previously determined algorithm, based upon the image data output from the image-capturing device.
With the above described digital camera, a white cap is fitted upon the photographic lens or an image is captured of a photographic subject which has uniform white brightness, a white balance adjustment coefficient is calculated based upon the image data output from a predetermined region of the CCD, and white balance adjustment is performed for the image data from all of the regions of the CCD using this adjustment coefficient which has been calculated. Generally the light gathering rate for the CCD is reduced, since a large number of pixels (picture elements) are gathered in a specified area in order to increase the number of pixels, and therefore the area of the CCD for one pixel becomes small. In this connection, in order to increase the light gathering rate of the CCD, on chip micro lenses are formed upon the CCD.
With this type of CCD, the strength of the R, G, and B components of the light which is received by the photodiodes via the on chip lenses changes according to the angle of the light which is incident upon the on chip lenses. Accordingly, with a digital camera which photoelectrically converts light which passes through a photographic lens and is directed to a CCD, when the angle of incidence of the light upon the on chip lenses on the CCD changes due to change of the position of the pupil of the photographic lens, change of the aperture value, or change of the relative positions of the optical axis of the photographic lens and the CCD, it is easy for the strengths of the R, G, and B components of the light received by the photodiodes to change and for the white balance color adjustment to become improperly adjusted, and there is a danger that color loss and color fogged images may occur. In particular, a problem can easily occur with a digital camera of the high picture quality type in which the number of pixels can exceed 200,000.
The objective of the present invention is to propose a digital camera which is capable of sufficiently suppressing the phenomena of color loss and color fog by correcting changes in the output values of image data of the CCD generated by changes of the relative positions of the optical axis of the photographic lens and an image-capturing device such as a CCD or the like, and changes of the position of the pupil and the aperture value of the photographic lens, and to propose an image signal processing apparatus which is capable of this type of image processing.
In order to attain the above objective, a digital camera according to the present invention comprises: an image-capturing device which captures an image of a photographic subject which passes through a photographic lens, and outputs image data; a gain adjustment section which performs gain adjustment by multiplying the image data output from the image-capturing device by a predetermined gain; a detection section which detects an optical axis of the photographic lens upon the image-capturing device; and a gain adjustment coefficient calculation section which calculates a gain adjustment coefficient according to distance between a position of a part of the image on the image-capturing device and the optical axis, and the gain adjustment section performs gain adjustment of image data corresponding to the position of the part of the image on the image-capturing device using the gain adjustment coefficient.
In this digital camera, it is preferred that the gain adjustment coefficient is a function of distance between the position of the part of the image and the optical axis.
Also, it is preferred that a color filter provided upon the image-capturing device is further provided and the gain adjustment coefficient calculation section calculates a gain adjustment coefficient for each color for image data which have been captured via the color filter.
Also, it is preferred that the gain adjustment coefficient is a coefficient which compensates for reduction of light intensity based upon a cosine fourth power law for the photographic lens.
Also, it is preferred that the image-capturing device comprises a plurality of pixels; an on chip lens which increases light gathering rate is formed upon each of the plurality of pixels; and the gain adjustment coefficient is a coefficient which compensates for reduction of transmitted light intensity due to the on chip lens.
Also, it is preferred that the image-capturing device comprises a plurality of pixels; anon chip lens which increases light gathering rate is formed upon each of the plurality of pixels; and the gain adjustment coefficient is a coefficient which compensates for reduction of light intensity based upon a cosine fourth power law for the photographic lens, and for reduction of transmitted light intensity due to the on chip lens.
Another digital camera according to the present invention comprises: an image-capturing device which captures an image of a photographic subject which passes through a photographic lens, and outputs image data; a gain adjustment section which performs gain adjustment by multiplying the image data output from the image-capturing device by a predetermined gain; a detection section which detects an optical axis of the photographic lens upon the image-capturing device; and a gain adjustment coefficient calculation section which calculates a gain adjustment coefficient according to distance between a position of a part of the image on the image-capturing device and the optical axis, and lens photographic conditions of the photographic lens including at least one of pupil position and aperture value, and the gain adjustment section performs gain adjustment of image data corresponding to the position of the part of the image on the image-capturing device using the gain adjustment coefficient.
In this digital camera, it is preferred that the gain adjustment coefficient is a function of distance between the position of the part of the image and the optical axis.
Also, it is preferred that a color filter provided upon the image-capturing device is further provided, and the gain adjustment coefficient calculation section calculates a gain adjustment coefficient for each color for image data which have been captured via the color filter.
On a recording medium according to the present invention a program for image signal processing is recorded. The program comprises: a detection process in which an optical axis of a photographic lens located on an image-capturing device is detected in image data obtained by the image-capturing device; a gain adjustment coefficient calculation process in which a gain adjustment coefficient is calculated according to distance between an image position on the image-capturing device and the optical axis; and a gain adjustment process in which gain adjustment of image data corresponding to the image position on the image-capturing device using the gain adjustment coefficient.
In this recording medium, it is preferred that in the gain adjustment coefficient calculation process, a gain adjustment coefficient is calculated according to distance between an image position on the image-capturing device and the optical axis, and lens photographic conditions of the photographic lens including at least one of pupil position and aperture value.
A data signal according to the present invention transmitted via a communication line comprises a program for image signal processing. The program comprises: a detection process in which an optical axis of a photographic lens located on an image-capturing device is detected in image data obtained by the image-capturing device; a gain adjustment coefficient calculation process in which a gain adjustment coefficient is calculated according to distance between an image position on the image-capturing device and the optical axis; and a gain adjustment process in which gain adjustment of image data corresponding to the image position on the image-capturing device using the gain adjustment coefficient.
In this data signal, it is preferred that in the gain adjustment coefficient calculation process, a gain adjustment coefficient is calculated according to distance between an image position on the image-capturing device and the optical axis, and lens photographic conditions of the photographic lens including at least one of pupil position and aperture value.
An image signal processing apparatus according to the present invention that processes image data output by an image-capturing device, on which a plurality of pixels are arranged in two dimensions, and which captures an image of a photographic subject which has passed through a photographic lens, comprises: a gain adjustment coefficient calculation section that calculates a coefficient for adjusting a gain for each of the pixels according to distance between a pixel corresponding to a position of an optical axis of the photographic lens and each of the pixels; and a gain adjustment section which adjusts a gain for image data of each of the pixels by multiplying a predetermined gain by the coefficient.
In this image signal processing apparatus, it is preferred that the image data output by the image-capturing device includes data related to lens photographic conditions of the photographic lens including at least one of pupil position and aperture value; and the gain adjustment coefficient calculation section calculates a coefficient for adjusting a gain for each of the pixels according to distance between the pixel corresponding to the position of the optical axis and each of the pixels, and the data related to lens photographic conditions of the photographic lens including at least one of pupil position and aperture value.